Human races probably show genetic differences in resistance to certain diseases. The probable genetic basis for such differences, particularly relevant to the welfare of American minorities, will be investigated by three models: an experimental investigation of the genetic architecture of adaptive race formation in mimetic butterflies (known to have been produced by natural selection, as must have been differences in disease resistance in man); a laboratory study of the genetic basis of linkage modification under artificial selection in the silkworm; and a computer study of the conditions, particularly in relation to race formation, under which genetic factors modifying linkage will spread in the population.